Solar tube panel with dual-exposure heat absorption

ABSTRACT

A dual-exposure heat absorption panel is disclosed, which can be used in a solar receiver design. Generally, the heat absorption panel includes a tube panel through which a heat transfer fluid is flowed to absorb solar energy from heliostats that are focused on the tube panel. A structural support frame surrounds the tube panel. A stiffener structure runs across the exposed faces of the tube panel. The headers and other support structures on the periphery are protected by use of a heat shield. Different tube couplings are possible with this structure, as well as different stiffening structures at the headers. The heat shield can be shaped to create an open space, permitting focusing of sunlight on the edge tubes as well. A curtain can be used as an additional heat shield in certain scenarios.

BACKGROUND

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 61/560,527, filed on Nov. 16, 2011. The disclosure of thisapplication is hereby fully incorporated herein by reference in itsentirety.

The present disclosure relates broadly to the field of solar powergeneration used to produce electricity. More particularly, thisdisclosure relates to a dual-exposure or two-sided heat absorptionpanel, and a solar receiver including one or more of such panels. Thesesolar receiver designs can be used with Concentrated Solar Towertechnology, also known as Concentrating Solar Power (CSP) technology toharness the sun's energy to produce “green” electricity.

A solar receiver is a primary component of a solar energy generationsystem whereby sunlight is used as a heat source for the eventualproduction of superheated high quality steam that is used to turn aturbine generator, and ultimately produce electricity using the Rankinecycle or provide steam for other thermal processes.

Generally, the solar receiver is positioned on top of an elevatedsupport tower which rises above a ground level or grade. The solarreceiver is strategically positioned within an array of reflectivesurfaces, namely a field of heliostats (or mirrors), that collect raysof sunlight and then reflect and concentrate those rays back to the heatabsorbing surfaces of the solar receiver. This solar energy is thenabsorbed by the working heat transfer fluid (HTF) flowing through thesolar receiver. The reflective surfaces may be oriented in differentpositions throughout the day to track the sun and maximize reflectedsunlight to the heat absorbing surfaces of the receiver.

The solar receiver is an assembly of tubes with water, steam, moltensalts, or other heat transfer fluid (HTF) flowing inside the tubes. TheHTF inside the tubes of the receiver absorbs the concentrated solarenergy, causing the HTF to increase in temperature and/or change phases,so that the HTF captures the solar energy. The heated HTF is then eitherdirectly routed to a turbine generator to generate electrical power oris indirectly routed to a storage tank for later use.

Solar receiver designs typically include an arrangement of panels withvertically oriented tubes, i.e. tube panels, along with a supportstructure for maintaining the tube panels in place and other associatedequipment (pumps, pipes, storage vessels, heat shields, etc.). Inconventional designs, the solar receiver has a square, rectangular, orcircular cross-section (in a plan view from above). The tube panels arearranged on the exterior of the cross-section, so that the solar energyfrom the heliostats is directed at (and absorbed by) only one face of atube panel. This is illustrated in, for example, U.S. patent applicationSer. No. 12/605,241, which is entitled “Shop-Assembled Solar ReceiverHeat Exchanger” and is assigned to Babcock & Wilcox Power GenerationGroup, Inc., and which is hereby fully incorporated by reference herein.

In this regard, FIG. 1 is a plan view (i.e. viewed from above) of onesolar receiver design 100 discussed above, which has four tube panels110, 120, 130, 140, arranged as a square. Each tube panel has oneexterior face 112, 122, 132, 142 which is exposed to solar energy fromheliostats, and one interior face 114, 124, 134, 144 which is notexposed to such solar energy.

The interior non-absorbing face of a tube panel usually has a buckstaysystem that supports the tube panels against high wind, seismic forces,and thermally induced forces. The buckstay system typically includes “I”beams or other structural steel shapes that are clipped onto the tubepanel in such a way that the tube panel can expand independent of thesupport structure itself and independent of the other tubes and panels.Clips are usually welded to the tubes so that the tube panel can moverelative to the stationary support structure when heat is applied to thetubes, yet the support structure can still provide rigidity to the tubepanel. On a solar receiver, the tubes in the tube panel are not weldedtogether along their axes (i.e. membrane construction) as in a fossilfuel fired boiler, but are of loose construction. This allows the tubesto expand independently of each other when heat is applied. As a result,each tube must have a clip to attach to the buckstay at a supportelevation.

One problem that results due to only one face of a tube being exposed tosolar energy is that a temperature differential arises between theexposed hot face and the non-exposed cold face. This results indifferential expansion between the hot and cold faces of the tube, whichcauses the tube to bow. The severity of bowing depends on the magnitudeof the temperature differential and the rigidity of the tube panel.Because the clip connecting the tube to the buckstay keeps the tube inplace at the support elevation, bowing occurs between supportelevations. This creates high compressive stress on the heated side ofthe tube at each support elevation.

Due to daily heating and cooling of the tubes during startup, shutdown,and cloud passages, such stresses are cyclic, which can eventually leadto fatigue failure. For receivers that use molten salt as the HTF,impurities in the molten salt can also cause corrosion, which can beexacerbated where stress is located.

BRIEF DESCRIPTION

The present disclosure relates, in various embodiments, to heatabsorbing tube panels and solar receivers incorporating such panels thatare exposed to solar energy on two opposite faces. Compared to panelsthat absorb energy on a single face, heat absorption on two faces canreduce the temperature differential between the hot face and the coldface and therefore provide more uniform tube temperature around thecircumference of the tube. This results in significantly reduced thermalstresses in the tube and lower potential for tube failures. With lowertube stresses, the risk of failure due to stress corrosion is alsoreduced. Also, for a given panel size the available heat absorbing areais doubled compared to a single side heated panel. The combination ofreduced stresses and doubled absorbing area results in a panel that canaccept more than twice as much solar energy, significantly increasingthe efficiency of the panel. The solar receivers comprise an arrangementof heat transfer surfaces, a heat transfer fluid system structurally andfunctionally interconnected thereto, a vertical support structure, and astiffener structure. Various structural features and other additions arealso described herein.

Disclosed in embodiments herein is a dual-exposure heat absorptionpanel, comprising a tube panel and a structural support frame. The tubepanel comprises a plurality of vertical tubes for conveying a heattransfer fluid. The tubes are interconnected by at least one upperheader and at least one lower header. The tube panel has a first exposedface, an opposite second exposed face, an upper edge, a lower edge, afirst side edge, and a second side edge. The structural support frameruns along the upper edge, the first side edge, and the second side edgeof the tube panel. At least one tube in the tube panel is connected tothe at least one upper header or the at least one lower header by arepair coupling surrounding the at least one tube and a prior headertube stub.

The repair coupling may be located behind heat shields mounted to thestructural support frame so that the repair coupling is not exposed todirect sunlight.

The dual-exposure panel may further comprise a first stiffener structurerunning from the first side edge to the second side edge across thefirst exposed face and the second exposed face of the tube panel at afirst support elevation.

In some embodiments, the stiffener structure is formed from a firstsupport assembly and a second support assembly, each support assemblyincluding: a support tube a horizontal flange extending from the supporttube and having a slot therein and a scallop bar engaging one or morevertical tubes of the tube panel and having at least one lug, thescallop bar engaging the horizontal flange by a pin passing through theat least one lug and the slot of the horizontal flange. The support tubeof each support assembly may have a different diameter from any tube inthe tube panel, and in some embodiments is larger.

The dual-exposure panel may further comprise a second stiffenerstructure running from the first side edge to the second side edgeacross the first exposed face and the second exposed face of the tubepanel at a second support elevation. In specific embodiments, the firstsupport elevation and the second support elevation are not located at amiddle section of the tube panel.

Also disclosed herein in different embodiments is a dual-exposure heatabsorption panel, comprising a tube panel and a structural supportframe. The tube panel comprises a plurality of vertical tubes forconveying a heat transfer fluid. The tubes are interconnected by atleast one upper header and at least one lower header. The tube panel hasa first exposed face, an opposite second exposed face, an upper edge, alower edge, a first side edge, and a second side edge. The structuralsupport frame runs along the upper edge, the first side edge, and thesecond side edge of the tube panel. The tube panel includes at least onetube joined to a header tube stub on either the at least one upperheader or the at least one lower header, an exterior diameter of theheader tube stub being greater than a central exterior diameter of theat least one tube. In more specific embodiments, an interior diameter ofthe at least one tube is the same as an interior diameter of the headertube stub.

Also disclosed herein in different embodiments is a dual-exposure heatabsorption panel, comprising a tube panel and a structural supportframe. The tube panel comprises a plurality of vertical tubes forconveying a heat transfer fluid. The tubes are interconnected by atleast one upper header and at least one lower header. The tube panel hasa first exposed face, an opposite second exposed face, an upper edge, alower edge, a first side edge, and a second side edge. The structuralsupport frame runs along the upper edge, the first side edge, and thesecond side edge of the tube panel. The structural support frameincludes a first heat shield framing the first exposed face of the tubepanel, an open space being present between the first heat shield and thetube panel.

Also disclosed herein in different embodiments is a dual-exposure heatabsorption panel, comprising a tube panel, a structural support frame, acurtain, and means for guiding the curtain. The tube panel comprises aplurality of vertical tubes for conveying a heat transfer fluid. Thetubes are interconnected by at least one upper header and at least onelower header. The tube panel has a first exposed face, an oppositesecond exposed face, an upper edge, a lower edge, a first side edge, anda second side edge. The structural support frame runs along the upperedge, the first side edge, and the second side edge of the tube panel.The structural support frame includes a first heat shield framing thefirst exposed face of the tube panel, the first heat shield including anupper face, a first side face, and a second side face. The curtain islocated on the upper face of the first heat shield above the tube panel.The means for guiding the curtain is located on the first side face andthe second side face of the heat shield.

The curtain may have a length sufficient to cover the entirety of thetube panel. The means for guiding can include rails or cables.Sometimes, a bottom edge of the curtain includes weights.

These and other non-limiting aspects and/or objects of the disclosureare more particularly described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which arepresented for the purposes of illustrating the exemplary embodimentsdisclosed herein and not for the purposes of limiting the same.

FIG. 1 is a plan (i.e. top) view of a conventional solar receiver designhaving a square orientation, with each tube panel having one exteriorexposed face and one interior non-exposed face.

FIG. 1A is a side cross-sectional view of a conventional tube panel witha light barrier and insulation.

FIG. 1B is a perspective view of the panel of FIG. 1A.

FIG. 2 is a first front view of a solar receiver of the presentdisclosure using a dual-exposure heat absorption panel having a limitednumber of tube passes. In this figure, heat shields and panel stiffenersupport structures are removed to provide an interior view.

FIG. 3 is a second front view of a solar receiver of the presentdisclosure using a dual-exposure heat absorption panel. In this figure,panel stiffener support structures are visible, and heat shields areremoved to provide another interior view.

FIG. 4 is an exterior front view of a solar receiver of the presentdisclosure using a dual-exposure heat absorption panel. Here, the heatshields are in place.

FIG. 5 is an exterior side view of a solar receiver of the presentdisclosure.

FIG. 6 is a plan view showing a tube panel and a stiffener structure forthe tube panel of the present disclosure.

FIG. 7 is a side cross-sectional view of a tube panel and a stiffenerstructure for the tube panel as depicted in FIG. 6.

FIG. 8 is a front view of the tube panel and stiffener structure asdepicted in FIG. 6.

FIG. 8A is a perspective view of the tube panel and stiffener structureas depicted in FIG. 6.

FIG. 9 is an enlarged front view of a tube panel without stiffenerstructure showing the tube panel having multiple tube passes, upperheaders, and lower headers.

FIG. 10 is a schematic showing fluid flow through the dual-exposure heatabsorption panel.

FIG. 11 is a side cross-sectional view of the upper header and the tubepanel, showing a possible repair coupling arrangement between anoriginal tube and a replacement tube.

FIG. 12 is a side cross-sectional view of the lower header and the tubepanel, showing a tube stiffening arrangement.

FIG. 13 is a front view of an alternative arrangement of the heatabsorption panel, wherein an open space is located between the heatshield and the tube panel.

FIG. 14 is a front view of the heat absorption panel showing a curtainarrangement by which the tube panel can be quickly covered.

FIG. 15 is a side view of the heat absorption panel of FIG. 14.

FIG. 16 is a front view depicting the lowering of a curtain to cover thetube panel of FIG. 14.

DETAILED DESCRIPTION

A more complete understanding of the processes and apparatuses disclosedherein can be obtained by reference to the accompanying drawings. Thesefigures are merely schematic representations based on convenience andthe ease of demonstrating the existing art and/or the presentdevelopment, and are, therefore, not intended to indicate relative sizeand dimensions of the assemblies or components thereof.

Although specific terms are used in the following description for thesake of clarity, these terms are intended to refer only to theparticular structure of the embodiments selected for illustration in thedrawings, and are not intended to define or limit the scope of thedisclosure. In the drawings and the following description below, it isto be understood that like numeric designations refer to components oflike function.

The modifier “about” used in connection with a quantity is inclusive ofthe stated value and has the meaning dictated by the context (forexample, it includes at least the degree of error associated with themeasurement of the particular quantity). When used with a specificvalue, it should also be considered as disclosing that value. Forexample, the term “about 2” also discloses the value “2” and the range“from about 2 to about 4” also discloses the range “from 2 to 4.”

It should be noted that many of the terms used herein are relativeterms. For example, the terms “interior”, “exterior”, “inward”, and“outward” are relative to a center, and should not be construed asrequiring a particular orientation or location of the structure.Similarly, the terms “upper” and “lower” are relative to each other inlocation, i.e. an upper component is located at a higher elevation thana lower component.

The terms “horizontal” and “vertical” are used to indicate directionrelative to an absolute reference, i.e. ground level. However, theseterms should not be construed to require structures to be absolutelyparallel or absolutely perpendicular to each other. For example, a firstvertical structure and a second vertical structure are not necessarilyparallel to each other.

To the extent that explanations of certain terminology or principles ofthe solar receiver, boiler and/or steam generator arts may be necessaryto understand the present disclosure, the reader is referred toSteam/its generation and use, 40th Edition, Stultz and Kitto, Eds.,Copyright 1992, The Babcock & Wilcox Company, and to Steam/itsgeneration and use, 41st Edition, Kitto and Stultz, Eds., Copyright2005,The Babcock & Wilcox Company, the texts of which are herebyincorporated by reference as though fully set forth herein.

The present disclosure relates to a dual-exposure or two-sided heatabsorption panel and to solar receivers incorporating one or moretwo-sided heat absorption panels. The panels are designed to accept heaton two opposite sides or faces, rather than on only one side or face.This can reduce tube failures due to fatigue or stress corrosion, andfor a given panel size the available heat absorbing area is doubledcompared to a single side heated panel. The panels may include one ormore stiffener structures or heat shields. Generally, the solar receiveris located at the top of a vertical support structure which rises abovea ground level or grade. The vertical support structure may be supportedfrom a base. The heat transfer surfaces advantageously comprise loosetangent tube panels, which allows for unrestrained thermal expansion ofthe tubes/tube panels in both the horizontal and vertical directions,thereby eliminating additional tube stresses. As is known to thoseskilled in the art, the sizes of tubes, their material, diameter, wallthickness, number and arrangement for the heat transfer surfaces arebased upon temperature and pressure for service, according to applicabledesign codes. Required heat transfer characteristics, circulationratios, spot absorption rates, mass flow rates of the working fluidwithin the tubes, etc. are also important parameters which must beconsidered. Depending upon the geographic location where the solarreceiver is to be installed, applicable seismic loads and design codesare also considered.

It should be noted that in some embodiments, molten salt is used as theheat transfer fluid (HTF) that is run through the absorption panel. Inthis regard, molten salt solidifies at approximately 430° F. (221° C.,494° K.). When the tube panel(s) of the solar receiver is not exposed tolight/heat, either intentionally at shutdown or unexpectedly due to aheliostat field malfunction, the molten salt can quickly cool and formplugs. Plugged tubes can cause delays at start up and could lead to tubefailures. Thus, the ability to drain molten salt quickly is typicallypart of the solar receiver design. The valves and additional piping forsuch draining may not be depicted herein, but should be considered asbeing present. The present disclosure also contemplates the use ofwater, steam, or any other heat transfer fluid, with appropriatemodifications made to other components of the solar receiver.

FIG. 1A is a side view of a conventional tube panel 12 which utilizesone sided heat absorption, and FIG. 1B is an enlarged perspectiveexploded view of the tube panel. This one-sided heat absorbing tubepanel is used in the conventional solar receiver of FIG. 1. A reflectivemodular panel light barrier 36 is located behind the tubes 13 (i.e. thenon-exposed face of the tube panel) opposite the heat absorbing (i.e.exterior) side of the tube panel. The light barrier 36 is composed of anarray of metal sheets and may be coated with white paint or otherreflective material on the tube side to maximize reflectance of lightenergy back to the tubes and reduce operating temperatures of thebarrier plate. The light barrier is supported by a tube attachmentstructure, such as a buckstay support system 20. Behind the lightbarrier (i.e. further interior of the solar receiver) is the insulation38, which is covered by lagging. The light barrier is designed toprotect the insulation 38, support structure 20, and the interior partsof the solar receiver from rain and heat exposure that may travelthrough the gaps between the loose tangent tubes of the tube panels.

FIGS. 2-4 are various front views of a solar receiver design with adual-exposure or two-sided heat absorption panel, differing in thepresence or absence of certain structures and allowing for a bettercomprehension of the present disclosure.

In FIG. 2, a two-sided heat absorption panel 200 is visible. Theabsorption panel 200 includes a tube panel 210. The tube panel 210 has afirst exposed face 222 and a second exposed face 224 (not visible seeFIG. 5) opposite the first exposed face. The term “exposed” refers tothe fact that concentrated sunlight from heliostats can be directedagainst the face of the tube panel. The first face 222 and second face224 may also be referred to as exterior faces, which also refers totheir being able to receive concentrated sunlight from heliostats. Thefirst face and the second face are generally planar surfaces. The tubepanel 210 extends between an upper header 242 and a lower header 250.Put another way, the tubes in the tube panel are interconnected by atleast one upper header and at least one lower header. It should be notedthat in practice, the tube panel may include multiple upper headers andlower headers. The tube panel 210 also has an upper edge 212, a loweredge 214, a first side edge 216, and a second side edge 218. It shouldbe noted that in this view, one can see through the structure betweenthe tube panel 210 and the structural support frame 300.

A structural support frame 300 runs along the upper edge 212, the firstside edge 216, and the second side edge 218 of the tube panel. Thestructural support frame 300 includes a first vertical column 310, asecond vertical column 320, and an upper horizontal beam 330 extendingfrom an upper end 312 of the first vertical column to an upper end 322of the second vertical column. As seen here, the first vertical column310 is adjacent the first side edge 216, the second vertical column 320is adjacent the second side edge 218, and the upper horizontal beam 330is adjacent the upper edge 212 of the absorption panel. The tube panel210 is connected to the structural support frame 300 through the upperheader 242. Here, the tube panel is top supported. At least one panelsupport rod 202 extends between the structural support frame 300 and theupper header 242 three such panel support rods are shown here.

The structural support frame 300 rests upon a base platform 204, whichmay be considered as providing a platform for the absorption panel. Thebase platform 204 is attached to or located upon a tower 206.

Generally, a tube panel 210 requires at least one tube pass 240, anupper header 242, and a lower header 250. HTF flows from the inletheader to the outlet header (e.g. here the upper header can be the inletheader) and is heated in the tube pass by solar energy from heliostats.Each tube pass 240 includes at least one tube, and generally includes aplurality of such tubes. In FIG. 2, the tube panel is shown with aplurality of tube passes (here four). The tube panels and tube passescontemplated herein are of loose tube construction to allow independentdifferential expansion between tubes, reducing tube stresses. Theexposed faces of the tubes may be coated or painted to increase/maximizeheat absorption, for example with a special high temperature blackpaint. Adjacent tube passes are arranged so that heat transfer fluidflows upward through one tube pass and down through another tube pass ina serpentine manner. Various fluid flow arrangements may be used tofacilitate draining of the HTF and minimize the number of vent and drainvalves. Arrows here illustrate one such fluid flow arrangement.

In FIG. 3, two stiffener structures are shown. Each stiffener structurepreferably runs from the first side edge 216 to the second side edge 218across the first face 222 and the second face 224 of the tube panel.Here, a first stiffener structure 401 is located at a first supportelevation 225 and a second stiffener structure 402 is located at asecond support elevation 226. The two stiffener structures are arrangedin parallel. As explained further below, each stiffener structure isformed from two support assemblies, one support assembly on each face ofthe tube panel. Each support assembly includes a support tube. Here,support tube 400 is visible on this first face. The support tube 406provides stiffener structures on the second face.

Generally, the number of stiffener structures can depend on the maximumunsupported length of the tube panel that will resist wind and seismicloads. In this regard, the tube panel 210 can be considered as beingdivided into an upper section 230, a middle section 232, and a lowersection 234, which generally (but not necessarily) divide the exposedportion of the tube panel into equal sections along its height. Thefirst stiffener structure 401 is shown in the upper section 230, and thesecond stiffener structure 402 is shown in the lower section 234. Putanother way, the stiffener structures are typically not located in themiddle section. This keeps the stiffener structures out of the peak heatflux zone and reduces their operating temperatures. It is contemplatedthat the stiffener structures will include support tubes that will becooled by some heat transfer fluid, which could be the same as ordifferent from the HTF that is passed through the tube panel. Forexample, the use of oil or water can eliminate the potential for moltensalt freezing in the stiffener structure during startup and shutdown.Here, the stiffener structures are illustrated as being formed in partby a support tube 400 which is connected to the upper header 242 andlower header 250, which uses the same HTF as that passing through thetube panel 210. The stiffener structures 401, 402 are the portions ofthe support tube 400 that run across the face 222 of the tube panel 210.The circuitry is ultimately designed to minimize temperatures andstresses, allow independent thermal expansion of the stiffenerstructure, and minimize the potential for freezing of fluid duringstartup. The outer face of the stiffener structure can be painted orcoated to reduce/minimize heat absorption.

In FIG. 4, the structural support frame (not visible see FIG. 2) isshown with heat shields mounted to protect certain parts of the designfrom exposure to the concentrated sunlight coming from the heliostats.The structural support frame 300 is not visible in FIG. 4, but isvisible in FIG. 2. Here, a first heat shield 340 frames the first face222 of the tube panel 210. A second heat shield 360 (not visible seeFIG. 5) also frames the second face 224 of the tube panel. In thisregard, the heat shield 340 includes an interior edge 342 that forms awindow in the heat shield through which the tube panel 210 is visible.Dotted lines show the outline of the tube panel 210, the upper header242, and the lower header 250. As seen here, the interior edge 342 ofthe heat shield abuts the side edges 216, 218 of the tube panel, butcould also be arranged with a gap between the heat shield and side edgesof the tube panel to reduce spillage onto the heat shields. Each heatshield 340, 360 could also be considered as having an upper face, afirst side face, a second side face, and a lower face. The first heatshield and the second heat shield are generally made from aheat-resistant material. The heat shield(s) can also be coated orpainted with a reflective high temperature white paint todecrease/minimize heat absorption and/or operating temperature.

FIG. 5 is an exterior side view of the solar receiver design. The firstheat shield 340 and the second heat shield 360 are visible here. Theexposed first face 222 and second face 224 are also indicated. The base302 of the structural support frame is shown here as being wider thanthe apex 304 of the structural support frame this provides additionalstability. It should be noted that a heat shield 370 is also present onthe sides of the structural support frame 300.

As noted in FIG. 3, stiffener structures are used to support andstrengthen the tube panel. FIGS. 6-8A are different views of oneexemplary embodiment of a stiffener structure. FIG. 6 is a plan (i.e.top) view of the exemplary embodiment. FIG. 7 is a side cross-sectionalview of the exemplary embodiment. FIG. 8 is a front view of theexemplary embodiment. FIG. 8A is a perspective view.

Referring to FIG. 6, the stiffener structure 401 is formed from a firstsupport assembly 410 and a second support assembly 470, which arelocated on the opposite exposed faces of the tube panel. (Referring backto FIG. 3, the first support assembly 410 is part of the support tube400, and the second support assembly 470 is part of the support tube406.) Each support assembly 410 includes a support tube 420, horizontalflange 430, and scallop bar 440. The support tube 420 is contemplated tobe hollow and allow a cooling fluid to pass through. A horizontal flange430 extends from the support tube inwards towards the tube panel 210.The horizontal flange 430 has a slot 432 therein. As seen here, thehorizontal flanges 430, 472 on the two support assemblies are opposed toeach other. The scallop bar 440 has a contoured face that engages thetube panel 210, and lugs 448 on an opposite face. The scallop bar isconnected to the support tube by a pin 450 which passes through the lugs448 and the slot 432. The scallop bar is held snug (but not fixed)against the panel tubes 460 with pins 452 that pass through lugs 454that are welded to some of the panel tubes, and the scallop bar engagesone or more of the tubes. The lugs 454 holding the scallop bar 440between the tubes 460 and pins 452 are offset from the lug 448connecting the scallop bar 440 to the support tube 420. This allows thepanel tubes and scallop bars to thermally expand in unison in thevertical direction, independent of the relatively stationary (in thevertical direction) support assembly. A protective sleeve 446 can beplaced between the panel tube and the scallop bar as shown to protectthe tubes from wear and/or gouging if any relative motion (slidingcontact) occurs between the scallop bar and panel tubes. It is notedthat only one pair of flanges and lugs 430, 478 is depicted here, butadditional flanges and lugs may be present on each support assembly toresist panel twisting and maintain panel-to-panel alignment. Similarly,only one scallop bar 440 is shown attached to support tube 420, butmultiple scallop bars could be used along the support tube to stiffen asingle wide panel or multiple panels, for example, if there is asignificant difference in vertical thermal expansion between tubeswithin a panel or between panels, as desired. Also, each scallop bar 440could have multiple lugs 448. The stiffener structure can be supportedby the structural support frame (see FIG. 3). The support tubes can beattached or connected to the vertical columns of the support frame,though they are not shown here as such.

The stiffener structure allows for independent thermal expansion of theindividual tubes in the tube panel, as well as for independent thermalexpansion of the stiffener structure and the support tubes. The pin/slotarrangement between the scallop bar and the support tube permits thesupport tubes to thermally expand axially independently of the radialexpansion of the tubes in the tube panel. (Note the axis of the supporttube is perpendicular to the axis of the tubes in the tube panel.)

The support system described above allows the individual tubes 460 to bearranged in a tangent tube fashion with minimal gap between the tubes.This reduces energy loss from light passing through the gaps andtherefore increases receiver heat absorption and efficiency. Theindividual tubes 460 are seen here with their centers 462 along themidline 405 of the tube panel. Other variations on the tube layout arealso contemplated.

Referring now to FIG. 7, in some embodiments, the support tube 420 ofthe support assembly could have a different diameter 425 from thediameter 465 of any tube 460 in the tube panel to provide the supporttubes with additional stiffness and in order to stiffen the panel andshade the parts associated with the support assembly, thus reducing partoperating temperatures. In some embodiments, the support tube diameter425 is larger than the diameter 465 of any tube 460 in the tube panel.The support tube 420 can also be considered as having an inner face 422and an outer face 424, the outer face being exposed to reflectedsunlight from the heliostats. The outer face 424 of the support tube canbe coated or painted to decrease/minimize heat absorption and/oroperating temperature.

Referring to FIG. 3, at least three variations on the stiffenerstructures are specifically contemplated. First, the support tubes 400,406 that make up the stiffener structures 401, 402 are illustrated asbeing connected to the upper header 242 and the lower header 250, sothat they use the same HTF as flows through the tube panel 210. However,other embodiments are contemplated in which the support tubes use adifferent cooling fluid. This could be accomplished, for example, byconnecting the support tubes to separate headers. Second, support tube400 is illustrated here as contributing the support assembly to bothstiffener structures 401, 402. In other embodiments, the stiffenerstructures could be made using separate support tubes. For example, asupport tube could run across the first support elevation 225, but wouldnot run back across the second support elevation 226 a different supporttube could be used for the stiffener structure at the second supportelevation if necessary. Third, as illustrated here a stiffener structure401 uses two separate support tubes 400, 406. Other embodiments arecontemplated where only one support tube is used for the stiffenerstructure. This could be done, for example, by forming the support tubeas a rectangular torus that surrounds the tube panel. This singlesupport tube would provide the stiffener structure 401 adjacent to thefirst face of the panel and then wrap around the panel at the sameelevation and provide the stiffener structure adjacent to the oppositeface of the tube panel. This could be done at the second stiffenerstructure elevation 402 also by the same support tube or a differentsupport tube.

It is also noted that in FIG. 3, each support tube connects to the upperheader and the lower header on the same side of the tube panel. Forexample, support tube 400 connects to both the upper header 242 and thelower header 250 along first side edge 216. It should be understood thatthis may differ. For example, if only one stiffener structure ispresent, support tube 400 could connect to the upper header 242 alongfirst side edge 216, then cross the first face and connect to the lowerheader along second side edge 218.

FIG. 9 is an enlarged front view of the tube panel, with the stiffenerstructure removed. Generally speaking, the tube panel 500 includes aplurality of tube passes 510, depicted here with four tube passes. Eachtube pass comprises one or more tubes 512 which are parallel to eachother. The tubes 512 pass between an inlet header 514 and an outletheader 516 to form a body or wall 537 upon which the focused solarenergy from the heliostats can be directed. The tube passes 510 areinterconnected using jumper pipes 502. The tube passes 510 are organizedin a vertical or axial direction, such that the heat transfer fluidflows in an alternating up-down direction through the tube passes, whichis indicated with arrows 505. This change in flow direction is referredto herein as a serpentine flow path.

The flow path begins at inlet 504 and ends at outlet 506. It should benoted that if there is an even number of tube passes 510, the inlet 504and the outlet 506 may be located along a common edge 508 or 544 of thetube panel 500. Alternatively, the inlet 504 and outlet 506 can belocated on opposite edges 508 and 544 of the tube panel 500 when an oddnumber of tube passes is used. In other words, the inlet and the outletcan be independently located at the top edge 544 or the bottom edge 508,as required by the design of the receiver. As depicted here, the inlet504 and the outlet 506 are both located along the top edge 544.

An inlet header is defined as such relative to the direction of flow.Thus, for tube pass 530, header 531 is considered the inlet header andheader 532 is considered the outlet header. However, for adjacent tubepass 540, header 542 is considered the inlet header and header 541 isconsidered the outlet header. The headers of the tube passes can also bedesignated as upper headers 531, 541, 551, 561 and lower headers 532,542, 552, 562 wherein the upper headers are located above the lowerheaders. Put another way, one set of headers 532, 542, 552, 562 islocated in lower plane 508, and the other set of headers 531, 541, 551,561 is located in an upper plane 544.

Referring again to tube pass 530, the tubes 536 form a body 537. Thetubes are closely spaced and parallel to each other. The upper header531 has a width 533, and the lower header 532 has a width 534. In somecontemplated embodiments, and as illustrated here, the body 537 can havea width 538 that is greater than the header widths 533, 534. In otherwords, the body 537 may be wider than the lower header 532 and the upperheader 531. The width is measured in the horizontal direction. The lowerheader and the upper header of each tube panel are the same width. Theratio of the body width 537 to the width of the lower header or upperheader 532, 531 can at least 1.01:1, and may range from 1.01 to 1.5.This permits the spacing between edge tubes in adjacent panels to be thesame as the close spacing between tubes within a panel. In suchembodiments, the upper headers of adjacent tube panels would belaterally separated from each other. The lower headers of adjacent tubepanels would also be laterally separated from each other. This may allowthe tube panels to expand differentially with respect to each otherbecause they are operating at different temperatures.

FIG. 10 is a schematic diagram illustrating fluid flow through thedual-exposure heat absorption panel 600. Initially, a riser 670 providescold fluid to an inlet vessel 660 from cold storage tank 652. Forexample, “cold” molten salt may be pumped from the cold storage tankhaving a temperature of about 550° F. An inlet pipe 672 fluidly connectsthe inlet vessel 660 to the tube panel inlet 674. The jumper pipes 696between tube passes is also illustrated. An outlet pipe 678 fluidlyconnects the tube panel outlet 676 to an outlet vessel 662. The heattransfer fluid (HTF) can flow from the inlet vessel 660 through the tubepanel 684 to the outlet vessel 662. A downcomer pipe 688 leads from theoutlet vessel 662 back down to grade, where the “hot” fluid can flowinto hot storage tank 650.

The inlet vessel 660 is optional and not required, which is indicated bythe use of dotted lines, for example if the heat transfer fluid issteam/water. The outlet pipe 678 and outlet vessel 662 are also optionaland not required, which is indicated by dotted line. Without an outletvessel, the HTF flows from the tube panel outlet 676 directly to thedowncomer pipe 688 via outlet pipe 691. A bypass line 690 also connectsthe riser 670 to the downcomer pipe 688. If desired, this bypass flowpath can prevent the HTF from flowing through the tube panel 684.

This completes the energy collection process. The stored thermal energyin the heat transfer fluid can be used to generate steam andelectricity. This is done by, for example, pumping the hot HTF from thehot storage tank 650 through the shell side of a heat exchanger 654.Water enters the tube side of heat exchanger 654 and is converted tosteam. The steam can be sent to turbine 656, which drives an electricalgenerator 658. The cooler HTF leaving the heat exchanger then returns tothe cold storage tank 652, where it is pumped to the receivers to repeatthe energy collection process described above.

For a molten salt receiver, the tube panels must be fully drainable andventable. The receiver is usually drained when not in use, at sunset, orwhen available solar energy is too low. Molten salt solidifies atapproximately 430° F. (221° C., 494° K.). If not drained, the salt canfreeze inside the tubes, plug the receiver, and could rupture the tubes.As seen here, the solar receiver can include a vent valve 692 for eachindependent flow path which are both vented through the top of thedowncomer pipe 688. The vent valve is typically located near the top ofthe downcomer pipe 688, and the vent piping 694 is also illustratedconnecting the flow path to the downcomer pipe. One drain valve 697 istypically provided for each pair of tube passes, and is located beneaththe tube passes. The drain piping 698 is also illustrated, and connectsto the downcomer 688 so that fluid present in the tube panel drains andflows into the downcomer pipe 688. The vent valves and drain valves areautomated.

It should be noted that in FIG. 10, the various pipes are illustrated asbeing relatively straight fluid paths. However, it will be appreciatedby those skilled in the art that their actual design in terms ofarrangement and length will be determined by the degree of flexibilityrequired to accommodate expected motions caused by thermal expansion andcontraction during operation of the solar receiver. It is thus likelythat additional bends or length may be necessary to provide suchflexibility.

One problem with traditional solar receiver arrangements that have onlyone exposed face is that there is only limited access to the tubes inthe tube panels if a tube should fail. Referring back to FIG. 1, suchsolar receivers typically have panels around 360° of a supportingstructure, which leaves only one side for access to the tube panel (i.e.the interior side). In addition, referring to FIG. 1A and FIG. 1B, thepresence of the insulation and the light barrier increases themaintenance time needed to complete any repairs to the tube panels. Thetwo-sided heat absorption panel of the present disclosure allowsmaintenance access along the upper edge, lower edge, and two side edges,providing access around 360° of the tubes for removing a failed tube andinserting a new replacement. In addition, the tube-to-tube weld betweenthe headers and the tube panel is located within the heat-shielded area(see FIG. 5). This is desirable to reduce tube temperature due to fillermaterial in the new weld being thicker than the tube wall.

Traditional solar receivers typically use a tube-to-tube butt weld ofvery thin tubes. Because the new/repair weld is out of the concentratedsunlight, different tube couplings can be used. One such repair tubecoupling is seen in FIG. 11. This repair coupling is significantlyeasier to weld when replacing a failed tube. The header 750 is shownhere, with a header tube stub 760 from the prior (failed) tube extendingfrom the header. The prior header tube stub ends at a line 762, whichcan be cut in the field depending on the failure location of theoriginal tube. The prior tube stub is a portion of the former existingtube that did not fail. The new replacement tube 780 is abutted to thefield-cut line 762. A repair coupling 770 is used to surround the endsof the tube stub and the replacement tube, similar to inserting the twotubes into a cylindrical sleeve. Field welds can then be used to jointhe repair coupling 770 to the tube stub 760 and the replacement tube780 respectively (e.g. using a fillet weld). This repair coupling 770 islocated behind a heat shield, and is not exposed to the sunlight fromthe heliostats.

The tube panel can be stiffened using different means, such as thestiffener structure seen in FIGS. 6-8. Another stiffening structure canbe located in the heat-shield protected sections of the absorptionpanel. This is shown in FIG. 12. The lower header 250 is depicted hereas having a header tube stub 720. The header tube stub has an exteriordiameter 722 and an interior diameter 724. Also illustrated is a walltube 700 in the tube panel. The tube has an exterior diameter 712 and aninterior diameter 714. The interior diameter 724 of the tube stub 720 isthe same as the interior diameter 714 of the tube 710. However, theexterior diameter 722 of the tube stub is larger than the exteriordiameter 712 of the tube. In other words, the wall of the tube stub 720has a thickness 707 that is greater than the thickness 705 of the wallof the tube 710. The tube stub 720 and the tube 710 are welded togetherusing a fillet weld. Put another way, there is a discontinuous change inthickness. The heavier and thicker wall tube would increase the rigidityof the tube panel between the upper header and the lower header,permitting longer light exposed sections for the tube panel.Additionally, any support clips or welds could be larger and strongerdue to the thicker tubes.

FIG. 13 presents an alternative heat shield design. In FIG. 4, theinterior edge 342 of the heat shield 340 abuts the side edges 216, 218of the tube panel. Here, a gap or an open space 201 is present betweenthe side edges 216, 218 of the tube panel and the interior edge 342 ofthe heat shield. Such an open space creates a free-standing tube panel.This arrangement allows the heliostats to be focused more uniformlyacross the width of the tube panel, which generally requires someheliostats to be focused towards the edges of the tube panel. The openspace provides a buffer that reduces spillage of concentrated sunlightupon the heat shields. Instead, the concentrated sunlight can passthrough the open space, though it would be considered an energy loss. Inmore detail, the interior edge 342 of the heat shield includes an upperedge 344, a lower edge 346, a first side edge 348, and a second sideedge 350. An open space 201 is present between the interior side edges216, 218 of the heat shield and the side edges 348, 350 of the tubepanel. In particular embodiments, the open space has a width of at least1% of the width of the tube panel. Support tubes 400 and 406 are alsovisible here.

Another consideration in designing a solar receiver is a scenario inwhich heat transfer fluid (HTF) ceases to flow through the tube panel,for example by loss of plant power or loss of the pumps used to move theHTF through the solar receiver. In this scenario, the heliostats are allstill focused on the tube panel(s) of the solar receiver. The heliostatscannot be instantaneously defocused, and without HTF flow the high heatflux can quickly overheat the tube panel.

In a prior solar receiver known as Solar Two, which was operated fromJanuary 1998 to April 1999, inlet and outlet HTF vessels were used asbuffers. The inlet vessel was pressurized with compressed air at apressure high enough to continue flowing HTF contained within the inletvessel through the tube panels long enough to allow the heliostats to bedefocused off of the receiver.

FIG. 14 and FIG. 15 depict another arrangement which is permitted withthe solar receiver designs of the present disclosure. Here, the heatshield 340 includes an upper face 352, a first side face 354, and asecond side face 356. Again, a window or aperture 355 is present withinthe heat shield through which the tube panel 210 is visible. A curtain750 is located on the exterior of the upper face 352 of the first heatshield above the tube panel 210. Here, the curtain is rolled up in astowed position. The curtain can be made from a high temperatureresistant material, such as a ceramic blanket. Means 752 for guiding thecurtain are located on the first side face 354 and the second side face356 of the heat shield. As seen in FIG. 15, a curtain can also belocated on the second side on the second heat shield 360. Support tubes400 and 406 are also visible in FIG. 14.

When a trip condition exists, the curtain would be released and fall infront of the tube panel to block the concentrated sunlight coming fromthe heliostats. This would protect the tube panel from overheating untilthe heliostats could be defocused off of the receiver, eliminating theneed for an inlet vessel. One benefit of this solar receiver design isthat the edges of the curtain can be positively guided to pull thecurtain down and keep the curtain from blowing in the wind, which coulduncover portions of the tube panel. Here, the curtain can extend beyondthe width of the tube panel. Thus, the edges of the curtain can beguided, for example via rails (like a garage door) or using guidecables. Here, the guidance means is shown as a path 758 through the heatshield, with cables attached to the curtain. This also protects themechanism for driving the curtain down over the tube panel. For example,the bottom edge of the curtain may be weighted. Alternatively, cablescould be used to pull the curtain down from the sides.

FIG. 16 is a front view illustrating the lowering of the curtain. Thecurtain 750 is illustrated as being lowered about halfway down. Thebottom edge of the curtain is weighted (reference numeral 754). Guidecables 756 are running down the cable paths 758, and are attached to thebottom corners of the curtain.

The present disclosure has been described with reference to exemplaryembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the present disclosure be construed asincluding all such modifications and alterations insofar as they comewithin the scope of the appended claims or the equivalents thereof.

1. A dual-exposure heat absorption panel, comprising: a tube panelcomprising a plurality of vertical tubes for conveying a heat transferfluid, wherein the tubes are interconnected by at least one upper headerand at least one lower header, and wherein the tube panel has a firstexposed face, an opposite second exposed face, an upper edge, a loweredge, a first side edge, and a second side edge; and a structuralsupport frame that runs along the upper edge, the first side edge, andthe second side edge of the tube panel; wherein at least one tube in thetube panel is connected to the at least one upper header or the at leastone lower header by a repair coupling surrounding the at least one tubeand a prior header tube stub.
 2. The dual-exposure panel of claim 1,wherein the repair coupling is behind heat shields mounted to thestructural support frame and is not exposed to direct sunlight.
 3. Thedual-exposure panel of claim 1, further comprising a first stiffenerstructure running from the first side edge to the second side edgeacross the first exposed face and the second exposed face of the tubepanel at a first support elevation.
 4. The dual-exposure panel of claim3, wherein the stiffener structure is formed from a first supportassembly and a second support assembly, each support assembly including:a support tube; a horizontal flange extending from the support tube andhaving a slot therein; and a scallop bar engaging one or more verticaltubes of the tube panel and having at least one lug, the scallop barengaging the horizontal flange by a pin passing through the at least onelug and the slot of the horizontal flange.
 5. The dual-exposure panel ofclaim 4, wherein the support tube of each support assembly has adifferent diameter from any tube in the tube panel.
 6. The dual-exposurepanel of claim 3, further comprising a second stiffener structurerunning from the first side edge to the second side edge across thefirst exposed face and the second exposed face of the tube panel at asecond support elevation.
 7. The dual-exposure panel of claim 6, whereinthe first support elevation and the second support elevation are notlocated at a middle section of the tube panel.
 8. A dual-exposure heatabsorption panel, comprising: a tube panel comprising a plurality ofvertical tubes for conveying a heat transfer fluid, wherein the tubesare interconnected by at least one upper header and at least one lowerheader, and wherein the tube panel has a first exposed face, an oppositesecond exposed face, an upper edge, a lower edge, a first side edge, anda second side edge; and a structural support frame that runs along theupper edge, the first side edge, and the second side edge of the tubepanel; wherein the tube panel includes at least one tube joined to aheader tube stub on either the at least one upper header or the at leastone lower header, an exterior diameter of the header tube stub beinggreater than a central exterior diameter of the at least one tube. 9.The dual-exposure panel of claim 8, wherein an interior diameter of theat least one tube is the same as an interior diameter of the header tubestub.
 10. A dual-exposure heat absorption panel, comprising: a tubepanel comprising a plurality of vertical tubes for conveying a heattransfer fluid, wherein the tubes are interconnected by at least oneupper header and at least one lower header, and wherein the tube panelhas a first exposed face, an opposite second exposed face, an upperedge, a lower edge, a first side edge, and a second side edge; and astructural support frame that runs along the upper edge, the first sideedge, and the second side edge of the tube panel; the structural supportframe including a first heat shield framing the first exposed face ofthe tube panel, an open space being present between the first heatshield and the tube panel.
 11. A dual-exposure heat absorption panel,comprising: a tube panel comprising a plurality of vertical tubes forconveying a heat transfer fluid, wherein the tubes are interconnected byat least one upper header and at least one lower header, and wherein thetube panel has a first exposed face, an opposite second exposed face, anupper edge, a lower edge, a first side edge, and a second side edge; anda structural support frame that runs along the upper edge, the firstside edge, and the second side edge of the tube panel; wherein thestructural support frame includes a first heat shield framing the firstexposed face of the tube panel, the first heat shield including an upperface, a first side face, and a second side face; a curtain on the upperface of the first heat shield above the tube panel; and means forguiding the curtain located on the first side face and the second sideface of the heat shield.
 12. The dual-exposure panel of claim 11,wherein the curtain has a length sufficient to cover the entirety of thetube panel.
 13. The dual-exposure panel of claim 11, wherein the meansfor guiding includes rails or cables.
 14. The dual-exposure panel ofclaim 11, wherein a bottom edge of the curtain includes weights.